PCC Oxidation

Alcohols are oxidized by pyridinium chlorochromate (PCC) to the corresponding aldehydes or ketones. They are not further oxidized to the corresponding carboxylic adds because the reaction was done in organic solvents, not in water. If water existed, the carbonyls would form aldehyde hydrates or ketone hydrates, which are then oxidized to acids. 

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Pauli exclusion principle (Section 1 1) No two electrons can have the same set of four quantum numbers An equivalent expression is that only two electrons can occupy the same orbital and then only when they have opposite spins PCC (Section 15 10) Abbreviation for pyndimum chlorochro mate C5H5NH" ClCr03 When used in an anhydrous medium PCC oxidizes pnmary alcohols to aldehydes and secondary alcohols to ketones... 

PCC (Section 15.10) Abbreviation for pyridinium chlorochro-mate CjHjNIT ClCr03. When used in an anhydrous medium, PCC oxidizes primary alcohols to aldehydes and secondary alcohols to ketones. 

The final stage is to introduce the C-13 hydroxyl group via PCC oxidation of 187 and subsequent reduction of the carbonyl group, and this completes the total synthesis of baccatin III (Scheme 7-58). The synthesized compound is identical to a natural sample of baccatin III in all respects, including optical rotation. 

Phenyl lithium attack on the cyclic carbonate convertes 202 to C-2 benzoate compound 203. PCC oxidation and subsequent NaBH4 reduction then furnishes the final baccatin III for the total synthesis of taxol (Scheme 7-65). 

Protection of alcohols. /-Butyldiphenylsilyl ethers (6, 51) are useful for pro-tecton of alcohols, but are more resistant to acid hydrolysis and fluorolysis than /-butoxydiphenylsilyl ethers. In contrast, /-butoxydiphenylsilyl ethers are relatively acid-stable but are readily cleaved by fluoride ion in CFLCU. The ethers are stable to most alkyllithiums and to Swern or PCC oxidation. 

The pyridinium chlorochromate (PCC) oxidations of pentaamine cobalt(III)-bound and unbound mandelic and lactic acids have been studied and found to proceed at similar rates.Free-energy relationships in the oxidation of aromatic anils by PCC have been studied. Solvent effects in the oxidation of methionine by PCC and pyridinium bromochromate (PBC) have been investigated the reaction leads to the formation of the corresponding sulfoxide and mechanisms have been proposed. The major product of the acid-catalysed oxidation of a range of diols by PBC is the hydroxyaldehyde. The reaction is first order with respect to the diol and exhibits a substantial primary kinetic isotope effect. Proposed acid-dependent and acid-independent mechanisms involve the rapid formation of a chromate ester in a pre-equilibrium step, followed by rate-determining hydride ion transfer via a cyclic intermediate. PBC oxidation of thio acids has been studied. ... 

Ketone rac-13 was transformed into the corresponding silylenolether and by Pd(II)-mediated Saegusa oxidation [14] into a, -unsaturated ketone rac-14. By alkylative enone transposition comprising methyl lithium addition and pyridinium chlorochromate (PCC) oxidation [15], rac-14 was finally converted into the racemic photo cycloaddition precursor rac-6. In conclusion, the bicyclic irradiation precursor rac-6 was synthesized in a straightforward manner from simple 1,5-cyclooctadiene (11) in nine steps and with an overall yield of 21%. 

In the case of the benzothiazole system, both aldehydes and mixed ketones have been synthesized by reduction or alkylation of the appropriate carbonyl precursors. The carbonyl compounds are in turn prepared from the benzothiazole-2-anion either directly by reaction with esters or indirectly by reaction with aldehydes followed by PCC oxidation (Scheme 154) (85H2467 91BCJ3256). 

More recently, the same group achieved a simple, highly stereocontrolled total synthesis of (+)-hirsutic acid (Scheme LXXIX) ". This chirally directed effort developed subsequent to reaction of dl-728 with (+)-di-3-pinanylborane, alkaline hydrogen peroxide oxidation, chromatography, PCC oxidation, and hydrogenolysis. The dextrorotatory hydroxy ketone 729 was nicely crafted into keto aldehyde 730 from which 720 was readily obtained. Once again, the Wacker oxidation played an instrumental role in annulation of the third five-membered ring. The remainder of the asymmetric synthesis was completed as before. 

The PCC oxidation of 3-amino-4,5-diphenyl-2(3//)-oxazolone 96 affords mono-diazobenzyl 286 as the primary decomposition product obtained via loss of carbon monoxide from the postulated A -nitrenolactam 285. Oxidation of 96 with tert-BuOCl/NEt3 at 108 °C results in a deep green solution of 285, which reacts with dimethyl sulfoxide (DMSO) to give the sulfoximide 288 (Fig. 5.68). ... 

Protection of primary alcohols p-Anisyl ethers are readily prepared from primary alcohols by the Mitsunobu reaction [P(C6H5)3 DEAD]. The ethers are stable to 3 N HC1 or 3 N NaOH at 100°, to Jones or PCC oxidation, and to LiAlH4. Deprotection is effected in 85-95% yield by oxidation with CAN in aqueous CH3CN. 

In the vast majority of cases, CH2CI2 is used as solvent in PCC oxidations. Occasionally, other solvents, including benzene,195 tetrahydro-furan,196 acetonitrile,197 chloroform,192 dioxane,198 hexane,199 acetone-CH2CI2200 or toluene,201 are used in PCC oxidations. The use of some of these alternative solvents may be advantageous in some substrates.202 Use of DMF tends to promote the over-oxidation of primary alcohols into carboxylic acids.203... 

Other less common accelerants for PCC oxidations include the addition of organic acids or Ac20, as well as sonication with ultrasounds or irradiation with microwaves. 

The application of ultrasound may substantially shorten the reaction time in PCC oxidations.221 Apparently, the ultrasound produces an erosion of the surface of the particles of PCC suspended in methylene chloride and, therefore, accelerate its interaction with the organic substrates.221a... 

It is claimed that the action of microwaves may very substantially accelerate PCC oxidations, resulting in reactions lasting a few minutes rather than hours.222 Micro-waves may be applied both to suspensions of PCC in a dichloromethane solution of the organic reactant or to the dust, resulting from thoroughly mixing the reactant and PCC in a mortar. 

During PCC oxidations, a dark viscous material containing reduced chromium salts is produced, and can interfere in the separation and purification of the product. Very often, solid particles consisting an inorganic material, such as silica gel,223 Celite ,224 Florisil ,225 magnesium sulfate226 or montmorillonite K10198 are added to PCC oxidations, so that the reduced... 

The work-up of PCC oxidations can be greatly facilitated by the use of the PCC polymeric derivative, poly[vinyl(pyridinium chlorochromate)].228 Filtration of the polymer and concentration of the organic solution allow an easy isolation of the product. 

Although in PCC oxidations, it is very common to add simultaneously to the reaction an accelerant, a buffer and a work-up-facilitator it is not common to employ simultaneously two materials belonging to the same kind, with the exception of the combination of the two accelerants molecular sieve and acetic acid, which are very often used together. 

Tertiary and secondary amines can resist the action of PCC, while an alcohol is oxidized.303 Even so, secondary amines are very often protected against PCC oxidations. 

Although little pursued in the literature, it can be anticipated that addition of one equivalent of BF3 Et20—or other acid would prevent the interference of amine functionalities in PCC oxidations. 

Although PCC oxidizes thiols to disulfides308 and sulfides to sulfoxides,309 it is possible to selectively oxidize alcohols in the presence of sulfides.310,311... 

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